1. Field of the Invention
The present invention generally relates to a method of and an apparatus for optically reading information from an optical recording medium, such as an optical disc for recording video image information, audio sound information and so on, and reproducing the record information, and more particularly to a method of and an apparatus for optically reading information from an optical recording medium which information recording density is improved to be high.
2. Description of the Related Art
It is desired to read a large volume of information within a short time period from an optical recording medium such as an optical disc, as the volume of the information to be read out therefrom is increased in a great degree. Further, in case of recording and reading a HDTV (High Definition Television) signal, it is required to record and reproduce a signal of high frequency. Namely, it is required to realize the wideband of the signal.
In order to increase the volume of the record information and cope with the wideband, it may be tried to record a signal to the optical disc by a higher information recording density. Namely, by increasing the number of the convex or concave portions (called a signal pit, hereinafter) including the information which is recorded to each unit length of the signal pit, the recordable information volume can be increased. Further, by making one piece of the signal pit smaller, the wideband recording is improved, and the number of the signal pits to be read in each unit time is increased in order to reproduce the signal pits recorded by the high density.
An optical reproducing apparatus including an optical pickup to read the information recorded in this kind of signal pits of the optical disc, may be provided with: a semiconductor laser of lateral single mode, which is provided with a point light source having a light emitting point with about 0.1 .mu.m diameter; a beam splitter for separating an irradiation light onto an optical disc from a reflection light from the optical disc; an objective lens for focusing the irradiation light from the semiconductor laser onto the optical disc; and a photodetector such as a pin photodiode for detecting the reflection light. Here, the wavelength .lambda. of the semiconductor laser and the numerical aperture NA of the objective lens are selected such that the relationship ".lambda./NA&gt;0.1 .mu.m" is fulfilled, so that the spot size of the semiconductor laser on the optical disc is determined by the so-called diffraction limit, and that the diameter becomes 1.22 .lambda./NA.
Each of the signal pits on the optical disc is formed in the convex or concave shape, and the signal pits are arranged in a line (called a signal track hereinafter) along a circumferential direction of the optical disc and in a spiral shape as a whole, so that one signal track is formed on the optical disc. The information is recorded such that the change in the length of one signal pit in the circumferential direction corresponds to the change in the modulation of the signal to be recorded.
This kind of optical disc is provided with a protection layer, a reflection film and a transparent resin substrate, wherein the signal pits are formed with the reflection film between the protection layer and the transparent resin substrate. In this case, the reading light from the semiconductor laser is irradiated onto the signal pits through the transparent resin substrate, and by reading the length of each signal pit in the disc rotating direction i.e. the circumferential direction of the optical disc, the information recorded on the optical disc can be reproduced.
The operation of this kind of optical disc is explained here.
The reading light e.g. the laser beam is emitted from the semiconductor laser, is deflected by the beam splitter, and is irradiated onto the signal pits of the optical disc while it is focused by the objective lens. When the laser beam spot is incident to the signal pit and is diffracted by the signal pit, the diffraction amount is varied by the size of the signal pit. By this variation in the diffraction amount, the intensity of the reflection light which is reflected toward the objective lens is also varied. In this manner, the reading light irradiated onto the signal pit is modulated by the form or size of the signal pit. The reflection light which is modulated in this way, is detected by the photodetector through the objective lens and the beam splitter, so that the information recorded on the optical disc can be read.
Further, an astigmatism is given to the reflection light form the optical disc, at the time of passing through the beam splitter. By this astigmatism, there is generated a difference between the light focusing position in the longitudinal direction of the light beam and the light focusing position in the lateral direction of the light beam. The photodetector is disposed at the middle position of those two focusing positions to receive the reflection light in an optimum condition.
In the optical pickup apparatus having the above explained construction, if it is tried to read the information of the optical disc which information recording density is made high by shortening the length of each pit and the interval of the adjacent pits, since the size and the interval of the pits are so small that the spatial frequency of the signal pits exceeds the so-called optical cut-off frequency (which is proportional to the value of NA/.lambda.). Accordingly, it becomes impossible to read the information.
Therefore, it becomes necessary to increase the reproduction spatial frequency of the optical pickup by increasing the optical cut-off frequency. In order to achieve this, the wavelength of the reading light emitted from the light source should be shortened, or the numerical aperture of the objective lens should be increased.
However, in order to shorten the wavelength of the light from the light source, in place of the presently available semiconductor laser, a gas laser or a laser using a non-linear optical element etc., should be employed, resulting in the increase of the cost and the increase of the size of the apparatus, which is a serious problem in a practical sense.
On the other hand, in order to increase the numerical aperture of the objective lens, the objective lens becomes so large that the production of the optical pickup may become very difficult. Further, when the shape of the light spot on the optical disc is greatly changed due to the inclination of the optical disc or the non-uniformity of the surface of the optical disc, such a phenomenon occurs that the signal pits around the signal pit to be detected are also detected. Thus, the production of the objective lens to overcome this phenomenon may be even more difficult.